Need more battery life for your iPhone 4? We've reviewed three of the top cases to find which is best.

The iPhone 4 is better than its 3G and 3GS siblings in terms of battery life, but it's still nowhere near enough for a traveller.

The standard battery lasts about eight hours, which is just enough for the phone to last until the nightly commute home on a normal work day, but when travelling (or even if you forget to charge the phone overnight) that limited battery life is annoying.

We've taken a look at three of the latest iPhone battery cases available to discover their pros and cons.

All the cases share some attributes in common: they all charge via a micro USB connector so you won't be using your iPhone dock cable any more. All the cases come with a micro-USB cord that you can plug into your existing iPhone wall charger. The case and the iPhone battery charge at the same time using the single micro-USB connector.

They all have a dock connector inside the case for your iPhone to plug into, and of course, this connects to the battery inside the case to keep your phone full of juice.

Mophie Juice Pack Air 4

Mophie is one of the better-known names in iPhone battery accessories, as it has been making them for several generations of iPhone.

Its latest Juice Pack Air certainly looks the part, with a silver plastic rim around the case mimicking the look of the iPhone 4's exterior metal antenna band.

It provides good protection for all sides of the iPhone, with the back fully covered and a protective ridge standing above the edges of the front screen so that if you drop it flat on a pavement the screen won't make contact.

It has holes in the case to allow direct access to the iPhone's metal buttons, which makes adjustments easy. The Energizer case, in comparison, makes you press through the silicon case, which requires more effort.

One annoying thing about the Juice Pack Air is that its bright, flashing charging status lights and micro USB charging connector are both at the bottom of the case, so no matter which way you lie it down, your bedroom will be turned into a disco overnight.

There's a charging on/off switch on the case, which is there to help preserve the life of the battery in the case. Mophie suggests you only switch it to charge when your iPhone is actually low on juice, as the battery case can only be recharged 500 times before its battery will start to fail (and of course, the more often you empty the battery into your iPhone, the more frequently you'll have to charge the case).

The capacity of the Mophie battery is 1500mAh, which gives a little more than twice the battery life of the iPhone 4's internal battery, which is rated at 1420mAh.

Padacs PowerCase for iPhone 4

Our favourite of the iPhone 4 battery cases we've looked at is Australian company Padacs' terrific PowerCase.

It comes in two capacities: a 1600mAh version for $49.95, which more than doubles the iPhone's own battery capacity of 1420mAh, and a 2200mAh version for $59.95.

It has holes in the side of the case to allow direct access to the iPhone's buttons.

The styling of the case is somewhat BlackBerry Torch-esque -- gloss black on all sides, with sculpted edges that disguise the heft of the battery. As a result, even the 2200mAh case looks pretty good despite its true bulk.

The fact that the case is sculpted and shiny also makes it the easiest of the three cases to slip in and out of your pocket.

The blue charging indicators are on the back of the case, but unfortunately also on a sculpted part of the case, so even if you lie it flat, it will illuminate your bedroom with a bright, rapidly flashing blue light overnight.

One quibble we had with the case is that by default it does not charge your iPhone -- you have to hold down a button on the back of the case for it to start charging. Presumably it has been designed like this so you don't use the case's battery unnecessarily for the same reason Mophie puts a switch on its case (see above), but it still feels like the decision has been taken out of your hands a bit.

The other minor downside of the PADACS cases is that the case rim sits flush with the iPhone 4 glass screen, providing very minimal protection for the screen if your drop it hard on its face on an uneven surface like bitumen.

You can buy the PowerCases for $49.95 or $59.95 (depending on capacity) directly from PADACS' website.

Energizer AP1201 iPhone 4 battery case

The Energizer iPhone 4 battery case is a soft silicon case with a battery implanted in a hard plastic back. As a result, it slips in and out of your pocket a little more easily than you might expect (as silicon is quite a grippy material). It is, however, still trickier than the other cases.

Its capacity is the lowest of the cases we looked at -- only 1200mAh, compared to the iPhone 4's own 1420mAh, so it doesn't quite double battery life.

The charging indicator for this case is on the back -- a series of small, deep blue lights. As a result, if you leave the case on its back overnight, the charging light won't keep you awake. You can get a read on remaining battery life by pressing a small button on one of the corners of the case.

One problem with the Energizer case is that it mostly encloses the phone, so if you want to use the volume buttons on the side or the lock button at the top of the phone, you have to squish them through the silicone, which requires considerably more effort than just pressing the buttons directly. The silent on/off switch is still exposed through a hole, though.

The case also picks up a lot of dust and fluff due to its silicone construction.

Unlike the Padacs or Mophie cases, the Energizer case constantly charges the iPhone -- there is no option to turn off its charging to keep the case battery in the best possible condition.

You can buy the Energizer AP1201 for $89.95 from the Australian importer's website - Mr Mobile.

Verdict: the best battery case for iPhone 4

In our testing of these three cases, the clear winner is the PADACS PowerCase. It provides greater battery life than any of the other cases, has a shape and texture that makes it super-easy to slip in and out of your pocket, and manages to disguise the bulk of the battery with clever industrial design.